Variable inductors



Sept. 22, 1959 I GElSER 2,905,912

VARIABLE INDUCTORS Filed Oct 21, 1955 INVENTOR. DA V/D I G'E/SER H/S Aromvsrs United States Patent VARIABLE INDUCTORS David T. Geiser, NorthAdams, Mass, assignor to Sprague Electric Company, North Adams, Mass, acorporation of Massachusetts Application October 21, 1955, Serial No.542,061

1 Claim. (Cl. 336-73) This invention relates to toroidal inductors, andmore particularly to toroidal inductors of the type whose inductance canbe selectively varied.

Variable inductors are used in various types of radio and higherfrequency applications. They may be used for intermediate-frequencytransformers, medium waveband coils, short wave coils, radio-frequencychokes, and various higher frequency applications such as in televisionservice, etc.

Q, or the Q factor, can be defined as two pi times the ratio of energystored to energy dissipated per half cycle of sine wave excitation. Itis indicative, therefore, of the operating efliciency of the coil. Foroptimum operating characteristics, a maximum Q should be obtained in aminimum volume or space occupied by the coil. Another important factoris economy of construction.

An object of this invention is to provide novel structures for variableinductors which are economical to manufacture and provide eflicientoperating characteristics.

In accordance with this invention, a magnetic core of endlessconstruction is wound with a flux inducing winding. A flux producingmeans is associated with said core for reacting with the flux induced bythe winding to form a leakage field about the core.

An element of flux-controlling material is disposed adjacent the core.This element is movably mounted to permit it to be selectively insertedwithin the leakage field to provide means for varying the inductance ofthe winding.

The novel features of the present invention will become apparent to oneskilled in the art from a reading of the following description, inconjunction with the accompanying drawings in which:

Fig. 1 is a somewhat diagrammatic representation of one embodiment ofthe present invention;

Fig. 2 is a somewhat diagrammatic representation of another embodimentof the present invention;

Fig. 3 is a somewhat diagrammatic representation of still anotherembodiment of the present invention;

Fig. 4 is a somewhat diagrammatic representation of a further embodimentof the present invention; and

Fig. 5 is a perspective View of a portion of another embodiment of thepresent invention.

Identical parts throughout the various figures are designated by thesame reference characters, followed by suflixes in indicative sequence.

In Fig. 1 is shown a variable inductor 8. A doughnut or toroidal-shapedendless core of magnetic material is provided. The core is made of anymagnetic material having a minimum permeability of approximately 2. Itmay be, for example, of powdered iron bonded together by a resin andpressed. Although doughnutshaped toroids are shown in the drawings,endless cores of any shape such as elliptical or angular may be utilized.to advantage. For higher frequencies, ferrites of the type described inU.S. Letters Patents No. 2,579,978,

No. 2,452,529 and No. 2,452,530 are particularly suita 2,905,912Patented Sept. 22, 1959 able. The core 20 may also be made of insulatedspirally wound foil or wire. For lower frequencies, ring-shapedlaminations may be bonded or clamped together to form a core.

A number of turns of conductive wire 12 are looped or wound around aportion of the core 10 to form a winding 13. The core, for example, maybe a toroid having an outer diameter of 0.60 inch, an inner diameter of0.27 inch, a cross-sectional area of 0.023 square inch, and an initialpermeability of 8. When the core is made of a ferrite having a highelectrical resistivity, the wire need not be specially insulated fromthe core. The winding provided by the conductive wire 12 may be formedof a number of equally spaced loops of conductive wire. The wire may bebare or covered by insulation such as those of the type known as Formvaror Ceroc-T. Litz wire of the braided or unbraided type also may be used.Formvar is a vinyl acetal enamel and magnet Wire coated with thisinsulation is obtainable from the Acme Wire Co. of New Haven,Connecticut. For a description of the structure and properties of theCeroc-T type wire, refer to U.S. Letters Patent No. 2,707,703. Thewinding may, for example, be made up of ten turns of No. 26 wire of theaforementioned types. The winding formed by wire 12 is connected acrossa source of alternating current 14 which may be R.F., U.H.F., etc.

A sheet-shorted turn 16 is disposed about another portion of the core10. The sheet-shorted turn is made of a sleeve of conductive material.This material may be, for example, conductive foil stock. The sleeve maybe formed by looping a sheet of foil about the core and then solderingthe outer edge. A sheet material printed or plated with a conductivepattern may also be used for the sheet-shorted turn. This sheet-shortedturn can be insulated or uninsulated from the core whether the core ismade of conductive material or non-conductive material.

A pair of protuberances or poles 18 and 20 can be provided on the innerportion of the core 10. These protuberances 18 and 20 are oppositelydisposed across the central aperture 22 of the core 10. These poles 18and 20 are disposed on an axis lying approximately between the winding13 and the sheet-shorted turn 16. They provide a path to help leakageflux transverse the aperture 22.

A slug 24 is mounted within the aperture 22 by means permitting axialmovement of the slug within the aperture. The slug 24 slides in achannel provided by protuberances 18 and 20. This slug is aflux-controlling member and therefore is made of either highly permeablematerial or highly conductive non-magnetic material. Insertion of theslug within the aperture therefore varies the effective permeability ofthe core. This in turn results in a change in the inductance imposed onthe source of alternating potential 14 by the variable inductor shown inFig. 1.

For a wide variation in inductance, the slug 24 may be divided into ahighly permeable portion 26 and a nonmagnetic highly conductive portion28. The highly permeable section may be, for example, soft iron and thenon-magnetic portion 28 may be brass or copper.

There may also be provided a cap or disc-like piece which bridges acrossa face of the core to effect a further variation in inductance by beingadjustably positioned with respect to the core. This cap can be of thesame inductance-controlling materials indicated above for the slug 24,and may also be formed integral with the slug or used without the slug,if desired.

Portions 26 and 28 of slug 24 may be selectively positioned within theaperture 22 by sliding the slug 24 to and fro. Where a fine adjustmentis desired, the slug and inner surfaces of the poles 18 and 20 may becooperatrively threaded" to'permit'axial positioning of theslug byrotation thereof. ,Variable inductors embodying the structure shown in lhave yielded a widerange as inrl'uctance variation atr'elatively high 'Qfactors. Y

' In- Fig. 2is -'shown avariahle inductor 81 similar to the inductor 8shown in Fig. l. The inductor 8a includes an end-lessor toroidal-core30,- similar in composition to toroidal core lttof Fig. l. The core 30,however,-ineludes a"recessed"slot"32 within which "a close-wound orlumped winding 34*is contained. The-winding is made up of wirelZa ofsimilar compositiontothat of' wire12 shown in Fig. 1. Protuberances lsw-and wa-are provided on the toroidal core'fit). These'protuberancesare similar in structure-and function to protuberances 18 and 20'shown-in Fig;1. Asheet-shortedturn 36 is'looped about anotherpor-tionof the core 3t). Thesheet-shorted turn 'is 'made 11pofsheet material of-a composition similar tothat-of the sheet-shorted turn' '1-6 -shown inFig. -1. Sheet-shorted turn 3a6 is, therefore, made up of copper foilstock. For'a half-inch nominal-(average) diameter toroidalcore30,-sheet-shorted turn 36- may be made of astrip of one mil copper-tape'%" wide. The-sides of the tape are slit to form-four parallelstrips. he central portion of the tape remains intact 'toform as-hor-ting band throughoutthe lengthof the resultant loop- The in-tact apermanent magnet material of the type known as Magnadur. Magnadur is amaterial which may be obtained from the Ferroxcube Corporation ofSaugerties, New York. It is manufactured by mixing oxides having theapproximate formula BaFe O The oxides are pressed or extruded into asuitable shape and sintered. This forms a hard, rather brittle substancehaving excellent permanentmagnet :qualities. .This permanent-magnet corehalf may be also formed of other permanent magnet materials, such .asAlnico. 'Ainico 1 maybe used; for example. It is a permanent magnetalloywhose average composition is as follows: iron62%, nickel-21%,aluminum-l2 and -cobalt-5%. A Winding 130 formed of wire 1 is providedon the core half 52. An inductance varying slug'24c is provided in theinner portion or aperture of the core 5% to vary the inductance in 4 asimilar manner to that shown and described in conjunction -withl'F-ig. 1. Alternatively-theme of :apermeable tuningslug 24c willicauseadivision of permanent magnet flux between .thecore isiliand the slug'24s, perrnitting ut'ilization of. somewhat lower frequency material inhigher frequency applications while giving 'greateriadjustableinductance range.

portion of the'tape is placed within the central portion er thecore andthe outer strips,--whic h rnay be designated as 36 ,86 36,-and'fifitrare-radially bent about the core to -form 'four' loopsjoined by theintact central portion. The -"-intactcentral portion of theturnis designated by reference character 38. The outerrnating edges ofthe looped strips 36 to '3 6 are conductively joined at their outeredge. They are-soldered, fonexample.

The. structure Sa-s-hown inZ-Fig. 2 provides a convenient method offorming-asheetashorted turn which-provides the-same -favorable operatingcharacteristics which are provided by the structure 8 shown inFigjl. Theclose wound winding 34 provides-Qfactors which are relatively higherthan those provided by even spaced winding I3,-'shown in Fig. 1. Inclose-winding the turns-may be wound at random ontopofeachother. Therecess 32 provides means for winding 2. core withoutmaking the structurebulky as well as leaving the protruding inner portionsof the core=t0act-as poles. Therecessing-of the sheet-shorted turn-is also seento havethese advantages. Various inductance varying structures may -be,'therefore,

moved into close proximity with the core w-ithout being interfered withby the flux-inducing winding. :In Fig.s3-'showna variable inductorstmcture 8b, includingzasheet-shorted turn 40 =whichis painted uponthewreiitlb. Thelcoreltlb is made eta highly-resistive ferrite :ofthettypeidescribed in conjunction wi-th Fig. 1. Protuberancesof thetypedesignated by reference charactersii and220.in:-Eig. .1: may beoptionally-provided.

"This .core ltlb is, .foriexample, :made of a fired equimolarimixtureof: nickel ;oxide ('NiO) I and ferrous oxide Fe O These .mixingweights-are accurate within 3 of 1%. This material is prepared in thenormal manner to form the/toroidal: core. Other.acceptableformulationsfor-the .core are .described.ini.previously mentioned Patents No.2,579,978,'1No. 2,452,529 and hid/2,452,530.

:The: sheet-shorted turn .is formed of highly conductive material. A?silver :turn provides. extremelyhigh conductivity. A:si'lveriturnisapplied to theicoreltlb by painting approximately. half .of. the corewith silver paint which is baked to harden. A winding .l'3b.islprovided' of I wire "12b; whichxis similar in composition and functionto that shownin Fig. l.

in-Fig. -4, avariable inductor structure 80 is shown. It is formed of acore'St) 'whichis made of two 'halves, 52

'aridl54. Thecore half 52 ismade of xa ferriteofsimilarconstructionto'the core-of-Fig.'l. The core'half 54, however, ismade-otamagnetized-permanent magnet'material. The core 11215 54 maybe ma'de forexample, of

Thisstructureprovldes :airelatively high amount of leakage flux betweenIthe poles orprotuberances 18c and 20c. Movement of slug 24c, therefore,varies the-inductance over a wideirange. This inductance-variation isaccomplished at.relatively high .Q factors over the entire range of avariation.

In .Fig. 5: iss'shown an inductance-varying .means leie which,ifortexample may beemployed to vary the inductance of 'a'toroidal core ina manner similar to that accomplishedibyportion 28 .of slug:24 shown inFig. -1. -'lhis inductance-varying means .may'be .describedas a cage.ilt 'iszmade upuof two vanes 82and:84,;crossed atrightQangles. -Thesevanes aremadeof nonmagnetic highly conductive material. These vanes, forexample, i areimade ofsheet brass. These vanes are-supported uponthe.:enlarged 'headffifi :of a rod-88. The rod :88Qpro- 'vides:meansforselectively inserting the cage within the aperturetof the core. 7

The sheet-shorted form of the present invention should.extendialongataleast about one-sixth of the-endless: pathilefined'tbythe toroid. .If'less inner circumferential-sheet.lengthibecomes.desirahle, itAis possible to formlseparate sheetloops:ofshighlyxonductive material bounding that portion of the toroidopposite the flux-inducing Winding. Somewhatflzetter Qsare :generallyobtained'when the lturnsioflthe winding are closely wound, thatisxconfined to only rabouttoneafourth of thetoroid pathor less.

More than one winding canlbe u'sed;.per core, as'forexample/to:makeltwoewinding transformers suitable for use asintermediatefrequency transformers in frequencymodulation receivers .orAM-receiving :sets, :preferably .rite,.shows.-at 47.25 .megacyclestpersecondan inductance -of 0.866-microhenry, a Q of 1 69, when tuned with acapacitance "of 13.25 micro-microfarads.

With a slug having a permeability of 12, the inductance can be raised toOJ984-microhenry, with'aQpf 1 39 and a'tuning capacitance ofl1.-42-micro-microfarads. The further addition of a cap with apermeability of 6.8 increases the inductance 'tg 1.033, lowers thetuning capacitance tQ 11.03 micro microfarads, and does not change theQ. In general, it is preferred to operate at frequencies at least ashigh as 10 megacycles per second, using wires having an effectivethickness corresponding to from No. 14 to about No. 30.

As many apparently widely different embodiments of this invention may bemade without departing from the spirit and scope hereof, it is to beunderstood the invention is not limited to the specific embodimentshereof except as defined in the appended claim.

I claim:

A variable inductor comprising a one-piece toroidal core of magneticmaterial having a central aperture, a winding looped about said core,means for applying an alternating current signal to said winding toinduce magnetic flux in said core, a thin layer of silver paint bakedabout the surface of approximately half of the said core lhavingextremely high conductivity for reacting with said flux induced in saidcore by said winding to cause a leakage field of flux to traverse saidcentral aperture a pair of protuberances oppositely disposed on theinner said aperture for widely varying the inductance of said winding.

References Cited in the file of this patent UNITED STATES PATENTS2,395,881

Klemperer Mar. 5, 1946 2,547,793 Spoor Apr. 3, 1951 2,666,187 KetchamJan. 12, 1954 FOREIGN PATENTS 476,422 Canada Aug. 28, 1951

